1. Field of the Invention
The present invention relates in general to the field of telecommunications, and more particularly, to a control circuit used in an enhanced Subscriber Line Interface Circuit (SLIC) for applying and removing ringing signals to a subscriber loop.
2. Description of the Prior Art
Subscriber line interface circuits are customarily found in the central office exchange of a telecommunications network. The SLIC weds the digital switching network of the central office exchange to a plurality of analog subscriber lines. The analog subscriber lines connect to subscriber stations or telephone instruments found at subscriber locations remote from the central office exchange.
The SLIC functions to supply power to a subscriber station and to transmit and receive voice signals between the digital switching network and the subscriber station.
Modern solid state SLICs are constructed using specialized integrated circuits. This construction dispenses with the need for inductive components in the analog end of the interface. The operating environment of the SLIC includes a necessity to provide high voltages and currents, used for analog voice transmission and substation signalling, as well as, low voltage digital logic signals used for the transmission of digital data between the SLIC and the digital switching network. The high voltage requirements of -48 V battery feed is accomplished using a specialized High Voltage Subscriber Line Interface Circuit (HVSLIC). Such as the HVSLIC circuit described in patent application Ser. No. 445,516, filed Dec. 4, 1989, entitled "High Voltage Subscriber Line Interface Circuit", having a common assignee with the present invention.
The translation of the analog voice signals to PCM encoded digital signals and the interface of the PCM digital signals between the line circuit and the digital switching system is accomplished using a specialized integrated circuit known as a CODEC/FILTER. The CODEC/FILTER converts analog voice signals received from a subscriber line and the HVSLIC to PCM encoded digital signals. Similarly, PCM encoded digital signals from the digital switching system are converted into analog voice signals for transmission on the subscriber line. One such device is the commercially available as the CODEC/FILTER COMBO.TM., TP30XX family of COMBO.TM. devices manufactured by the National Semiconductor Company.
The SLIC must also provide certain signalling and detection functions in order to allow the digital switching system to communicate with a subscriber station. These signaling and detection functions include ringing signal control, ring-trip and loop sense detection as well as detection of abnormal loop conditions.
These functions are normally provided by a low voltage companion circuit of the HVSLIC usually referred to as a Low Voltage Subscriber Line Interface Circuit (LVSLIC). One such LVSLIC circuit is described in U.S. Pat. Application Ser. No. 445,826, filed Dec. 4, 1989, entitled "Control Circuit For A Solid State Telephone Line Circuit", having a common assignee with the present invention. The LVSLIC communicates via a data and address bus with a central controller of the digital switching system. Information pertaining to the status of the SLIC and the subscriber line are transmitted from the LVSLIC to the central controller. Operating commands from the central controller are received by the LVSLIC for execution by the SLIC.
One of the functions provided by a LVSLIC is the application and deactivation of ringing signals to the subscriber loop. The LVSLIC under software control applies the ringing current to the subscriber loop from a ringing current generator and then waits for the subscriber instrument to go "off-hook". A detected "off-hook" or ring trip condition signifies that the subscriber has picked up the receiver and therefore the application of ringing current must cease. Such a ringing control circuit is described in U.S. Pat. Application Ser. No. 445,820, filed Dec. 4, 1989, titled "A Ring Trip Detector For A Solid State Telephone Line Circuit", having a common assignee with the present invention.
Presently known ringing control circuits suffer from false "off-hook" detection due to noise spikes induced onto the subscriber loop from external environmental sources. The induced noise is detected by the LVSLIC ring detector as an "off-hook" condition and the ringing terminated prematurely.
Further, many presently known ringing signal systems use relays to couple the ringing current into the subscriber loop. The metal contact closure of a relay bounces and chatters for several milliseconds after activation. This induces impulse noise into the adjacent subscriber loops which can degrade the quality of the audio path.
Therefore, it is advantageous to provide a ringing signal control system that can discriminate between an actual ring trip and noise pulses and also eliminates impulse noise from relay contact closure.
Presently available SLICs conform to a circuit architecture that connects in combination an integrated circuit HVSLIC, an LVSLIC, and CODEC/FILTER with discrete components that provide subscriber line configuration and protection. This SLIC combination connects a single subscriber line to the central office exchange.
Such a SLIC is described in U.S. Pat. Application Ser. No. 445,517, filed Dec. 4, 1989, entitled "A Solid State Telephone Line Circuit", having a common assignee with the present invention.
The SLIC just described is normally one circuit of a plurality of SLICs that are assembled on a line card. The line card connects a plurality of subscriber lines to the digital switching network. Typically, eight or more SLICs can be found on a single line card. However, each SLIC is susceptible to catastrophic damage due to the environment of the associated subscriber line, such as lightning strikes, power surges, etc. A failure of one SLIC circuit necessitates the replacement of the line card. It is advantageous therefore to be able to replace only those circuits of the line card that are damaged by the aforementioned environmental factors and not the entire line card.
Further, since the LVSLIC acts primarily as a local controller and signal detector between the central controller of the digital switching system and the HVSLIC and COMBO, a certain amount of economy in circuit components can be realized by removing the LVSLIC from the above mentioned combination. By placing the LVSLIC in a more central location the LVSLIC can provide control and detection functions to two or more HVSLIC and COMBO circuits. The central location also isolates the digital control from the high voltage associated with the HVSLIC.
Accordingly, it is an object of the present invention to provide a new and effective ringing signal control circuit for an enhanced subscriber line interface for applying and removing ringing current to a subscriber loop.